Prior inventors have designed a wide variety of inverter circuits. Inverter circuits are often found as a part of converter circuits which convert a DC voltage at a first amplitude to a DC voltage at another amplitude by intermediately converting the energy to periodic alternating current for accomplishing voltage level changes with a transformer.
Although inverter circuits may be single ended, for high power applications, e.g. exceeding 200 watts, conventionally they are of the push-pull type having a center tapped transformer. A DC source is connected to the center tap of the transformer and a pair of push-pull connected electronic switching means are connected to the ends of the primary for alternately applying the DC source to opposite havles of the primary. The electronic switching means are controlled by a control means connected to their inputs. The AC power is taken from the transformer secondary.
One problem with such inverter circuits is the power losses in the electronic switching devices which switch the primary current. These losses lower the efficiency of the circuit especially at high loads. A major cause of such power loss is the heat generated at the junctions of the switching device, such as a bipolar transistor, which is used to alternately switch the DC current through the transformer primary.
For example, a transistor typically may have a saturation voltage of 0.7 volts. If a 1,000 watt load is connected to the 120 volt output then a 12 volt DC source must supply at least 100 amps resulting in a junction loss of at least 70 watts.
Darlington connected transistor pairs have been popular as the switching elements for inverter circuits because their high gain permits use of very low switching power. However, a conventional Darlington pair has a saturation voltage twice that of a single stage device. For example, with reference to the above example, a 1,000 watt AC load would result in 140 watts of dissipation in the switching transistor.
There is therefore a need for a circuit for substantially reducing the dissipation and therefore increasing the efficiency of Darlington pair inverter circuits so that the high gain advantages of the Darlington pair can be utilized without the disadvantage of increased saturation losses and its resultant lower efficiency.
Inverter circuits which provide a useful AC power source require some regulation in order to maintain a constant output voltage over a wide range of load current.
Many types of regulation circuits are illustrated in the prior art. One type uses a pulse width modulation technique in which the width of the current pulses applied to the transformer is modulated to maintain a constant average AC output voltage as loading changes. In such a circuit the pulse width is responsive to the difference between a reference level and a signal which is derived and fed back from the AC output at the transformer secondary.
However, I have discovered that a defect exists in the prior art circuits because the signals which they feed back are not accurately proportional to the actual desired output condition which is to be regulated and maintained constant. In particular, prior art circuits feed back on AC signal which is proportional to the output, rectify it and then filter it to obtain its average value. However, it is RMS value of the output which should be regulated.
For a perfectly sinusoidal AC output voltage, the average signal obtained in that way is proportional to the RMS value of the AC output voltage. Therefore, for sinusoidal waveforms the average value may be used to regulate RMS value of the output because they are proportional. Of course, with a converter, in which a DC output is used, there is no problem. However, when pulse width modulation techniques are used and the output is AC, the distortion away from a sinusoid is substantial. Additionally, distortion of the output waveform changes as loading changes. With this distortion, the RMS value of the output is not proportional to the detected average value. Thus, the prior art circuits operate to regulate the average value and allows the RMS value to change. Since the equipment which is to be powered by the AC output is responsive to the RMS values, effective regulation is not maintained. For example, the RMS value of an AC output may vary by as much as 10% as loading increases even though the average value of the output voltage may be relatively constant.